We Burn as Many Calories as Hunter-Gatherers, So What Makes Us Fat?

Contrary to conventional wisdom, detailed energy measurements show that while hunter-gatherers may be more physically active than the rest of us, they don't burn more calories thanks to their lower metabolisms

We all know why Americans are so fat, right? We too much junk and we sit on our duffs all day.

Perhaps not, a team of international researchers now says. Their new study, examining energy expenditure among one of the world’s last remaining hunter-gatherer populations, seems to debunk our conventional wisdom — at least in part.

While we’ve always assumed that humans’ ancient ancestors must have been more active than today’s modern Westerners — with our office jobs, our cars and our TV sets to keep us sedentary — new measurements of actual energy expenditure are surprising. They show that people in traditional foraging societies do indeed participate in more physical activity, but that their total energy output is almost identical to that of today’s pudgy Westerners. This counterintuitive finding is explained by the foragers’ lower basal metabolic rate: they expend less energy while at rest, even when we compare people of the same size and age.

To gather the startling new measurements, researchers recruited 30 adults from the Hadza hunter-gatherer society, a small population living in the East African country of Tanzania. No society today is truly like the those of our ancestors tens of thousands of years ago, the researchers say, but the Hadza do share some important similarities with our Pleistocene-era forbears.

In particular, the Hadza maintain a traditional foraging lifestyle, hunting on foot using bows, small axes and digging sticks, and without modern tools like vehicles or guns. Their diet includes virtually no processed foods whatsoever. They live off of game that they hunt, and tubers, fruit and honey that they collect.

To measure energy use, participating Hadza adults wore GPS units to track how far they traveled each day. They also wore breath monitors while at rest and while walking to measure their metabolism in each state. And a measure of total energy expenditure was calculated from urine tests, which showed how quickly the study participants could eliminate a chemically altered water given to them to drink by the researchers. Then those measurements were compared to similar energy-expenditure measurements from 68 men and women who live in the U.S. or Europe, and also to data from farming communities in Bolivia, Nigeria and Gambia.

Contrary to even the researchers’ expectations, the scientists write, energy-expenditure measurements from the Hadza looked pretty similar to measurements elsewhere.

In fact, even though total energy expenditure did vary considerably by age, gender and by body size, as anticipated, when the researchers looked at men of the same age who each weighed, say, 130 lbs., there was no discernible difference by lifestyle group in total daily energy expenditure. On average, the Hadza were much smaller than the Westerners, both in height and in weight (130 lbs. was at the high end for Hadza males). But statistical analysis suggests that the basic relationship between energy spent and lean body mass — not including the Westerners’ extra fat pounds — was essentially the same across societies, and across people big and small.

Those results are all the more surprising because the Hadza did appear to expend much more energy in physical activity, as they hunted and foraged. But activity differences did not translate into differences in total energy use. What’s more, even among members of the same society, Hadza people who walked a long way each day did not have measurably higher total expenditure than individuals who did not walk so much. It seems that people’s metabolisms may compensate somewhat for activity level.

The new findings seem to contradict popular beliefs that weight management is simply a matter of balancing what we eat with enough purposeful physical activity.

“The similarity in [total energy expenditure (TEE)] among Hadza hunter-gatherers and Westerners suggests that even dramatic differences in lifestyle may have a negligible effect on TEE,” the authors conclude in their study, which is published this week in the journal PLoS One.

While the authors don’t look at diet in much detail, they add that their findings suggest that high energy intake — eating too much — is responsible for the West’s obesity epidemic, rather than too little energy expenditure. They do note, however, that physical activity is well-known to have many beneficial health effects in addition to any role in weight management.

Ultimately, what the study authors may have uncovered is that people are more similar than we previously realized. Across dramatically different societies and landscapes, human bodies function similarly.

“We hypothesize,” they write, “that [total energy expenditure] may be a relatively stable, constrained physiological trait for the human species, more a product of our common genetic inheritance than our diverse lifestyles.”